1. Field of the Invention
This invention relates generally to organoborane polyamine complex initiator systems and, more specifically, to systems in which the complex is carried in a polyol, and to systems that include polyisocyanate. The invention further relates to polymerizable compositions made therewith, particularly two-part adhesive compositions that have independent polyurethane/polyurea and acrylic components when cured. The adhesive compositions have excellent adhesion to a variety of substrates, especially low surface energy polymers.
2. Description of the Related Art
An efficient, effective means for adhesively bonding low surface energy plastic substrates such as polyethylene, polypropylene and polytetrafluoroethylene (e.g., TEFLON) has long been sought. The difficulties in adhesively bonding these materials are well known. See, for example, "Adhesion Problems at Polymer Surfaces" by D. M. Brewis that appeared in Progress in Rubber and Plastic Technology, volume 1, page 1 (1985). The conventional approaches typically function by: (1) increasing the surface energy of the substrate (to more closely match the surface energies of the substrate and the adhesive thereby promoting better wetting of the substrate by the adhesive) and/or (2) eliminating additives and low molecular weight polymer fractions in the substrate that can migrate to the substrate surface and adversely affect adhesion by forming a weak boundary layer.
As a result, the conventional approaches often use complex and costly substrate surface preparation techniques such as flame treatment, corona discharge, plasma treatment, oxidation by ozone or oxidizing acids, and sputter etching. Alternatively, the substrate surface may be primed by coating it with a high surface energy material. However, to achieve adequate adhesion of the primer, it may be necessary to first use the surface preparation techniques described above. All of these techniques are well known, as reported in Treatise on Adhesion and Adhesives (J. D. Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333 to 435). The known approaches are frequently customized for use with specific substrates. As a result, they may not be useful for bonding low surface energy plastic substrates generally.
Moreover, the complexity and cost of the presently known approaches do not render them particularly suitable for use by the retail consumer (e.g., home repairs, do-it-yourselfers, etc.) or in low volume operations. One vexing problem is the repair of many inexpensive everyday household articles that are made of polyethylene, polypropylene or polystyrene such as trash baskets, laundry baskets and toys.
Consequently, there has been a considerable and long felt need for a simple, easy to use adhesive that can readily bond a wide variety of substrates, especially low surface energy materials, such as polyethylene, polypropylene and polytetrafluoroethylene, without requiring complicated surface preparation, priming and the like.
While an adhesive that can bond low surface energy plastics is certainly advantageous, the commercial utility of such an adhesive would be enhanced if the components thereof could be combined in a convenient mix ratio. This would permit facile application of the adhesive using conventional adhesive dispensers without the need for laborious hand weighing and mixing of the different components. However, the convenient mix ratio should not come at the expense of significantly reduced storage stability or performance. Thus, there is not only a need for an adhesive that can bond low surface energy plastics, but a need for such an adhesive that can be readily blended in a convenient mix ratio without a material reduction in storage stability or performance.
It may be desirable for such adhesives to possess other attributes. For example, in certain applications it may be important for the adhesive to be tough, elastomeric or abrasion resistant, properties typically associated with polyurethane adhesives. In addition, it may be important to eliminate or reduce low molecular weight components in the adhesive that could migrate to the interface between the adhesive and the substrate. This could adversely affect adhesion by forming a weak boundary layer. Furthermore, if the low molecular weight components are solvent extractable, adhesive volume could be lost, voids could be created in the adhesive, and the adhesive and cohesive strength of the bond could be weakened if the bonded article was exposed to solvent. Also, low molecular weight components may be sensitive to water and their migration to the adhesive/substrate interface could decrease the hydrolytic stability of the adhesive bond.
As explained more fully hereinbelow, the organoborane polyamine complex initiator systems and the related compositions of the invention (which may include polyisocyanate, polyol, and acrylic monomer that can polymerize to polyurethane/polyurea acrylic adhesives) not only satisfy these demands but offer many other advantages.
Organoboranes such as tributylborane and triethylborane have been reported to initiate and catalyze the polymerization of vinyl monomers (see, for example, G. S. Kolesnikov et al., Bull. Acad. Sci. USSR, Div. Chem. Sci. 1957, p. 653; J. Furakawa et al., Journal of Polymer Science, volume 26, issue 113, p. 234, 1957; and J. Furakawa et al., Journal of Polymer Science, volume 28, issue 116, 1958). The organoborane compounds of the type described in these references are known to be quite pyrophoric in air which complicates facile use.
Chemical Abstracts No. 134385q (volume 80, 1974) "Bonding Polyolefin or Vinyl Polymers" reports that a mixture of 10 parts methyl methacrylate, 0.2 part tributylborane, and 10 parts poly(methylmethacrylate) was used to bond polyethylene, polypropylene and poly(vinyl acetate) rods.
U.S. Pat. No. 3,275,611 to E. H. Mottus et al. discloses a process for polymerizing olefinic compounds (e.g., methacrylate monomers) with a catalyst comprising an organoboron compound, a peroxygen compound, and an amine. The organoboron compound and the amine may be added to the reaction mixture separately or they may be added as a performed complex.
British Patent Specification No. 1,113,722 "Aerobically Polymerisable Compositions," published May 15, 1968 discloses the polymerization of acrylate monomers through the use of a free-radical catalyst (e.g., peroxides) and triarylborane complexes having the general formula (R).sub.3 B--Am wherein R is an aryl radical and Am is an amine. The resulting compositions are reportedly useful as adhesives.
Chemical Abstracts No. 88532r (volume 73, 1970) "Dental Self-curing Resin" and the full text paper to which it refers report that tributylborane can be made stable in air by complexing it with ammonia or certain amines and that the tributylborane can be reactivated with an amine acceptor such as an isocyanate, an acid chloride, a sulfonyl chloride, or anhydrous acetic acid. As a result, the complex can be used to polymerize blends of methyl methacrylate and poly(methylmethacrylate) to provide a dental adhesive.
A series of patents issued to Skoultchi or Skoultchi et al. (U.S. Pat. Nos. 5,106,928; 5,143,884; 5,286,821; 5,310,835; and 5,376,746) disclose a two part initiator system that is reportedly useful in acrylic adhesive compositions, especially elastomeric acrylic adhesives. The first part of the two part system includes a stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde.
The adhesive compositions are reportedly particularly useful in structural and semi-structural applications such as speaker magnets, metal-metal bonding, (automotive) glass-metal bonding, glass-glass bonding, circuit board component bonding, selected plastic to metal, glass, wood, etc. bonding, and electric motor magnets. Those plastics that may be bonded are not further described.
U.S. Pat. No. 4,043,982 (O'Sullivan et al.) describes a composition useful as an adhesive and which includes an acyl or silyl peroxide-type polymerization initiator (generally dissolved in a volatile solvent), a polymerizable acrylate-isocyanate monomer or oligomer, and an aryl amine polymerization initiator. The polymerizable acrylate-isocyanate monomer is the reaction product of an organic polyisocyanate with a polymerizable acrylate ester having a hydroxy or a primary or secondary amino group in the alcoholic moiety.
U.S. Pat. No. 4,721,751 (Schappert et al.) describes polyurea-polyurethane acrylate dispersions which are reportedly useful as adhesives and sealants. The materials can be prepared by reacting a polyisocyanate with a polyfunctional amine in the presence of a polyol, an ethylenically unsaturated diluent free of active hydrogens, and an active hydrogen containing polymerizable ethylenically unsaturated compound.
U.S. Pat. No. 4,731,416 (Saunders) describes a polyurethane-type adhesive comprising the reaction product of: (1) a true solution of a (a) copolymer of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid and a hydroxyalkyl ester of an .alpha.,.beta.-ethylenically unsaturated acid in (b) a polyahl having a molecular weight of at least 200; (2) an organic polyisocyanate; and (3) a polyahl chain extender having a molecular weight of less than 200.
U.S. Pat. No. 5,021,507 (Stanley et al.) describes an acrylic modified reactive urethane hot melt adhesive which can be obtained by adding a urethane prepolymer to low molecular weight polymers formed from ethylenically unsaturated monomers which do not contain active hydrogen.